1. Field of the Invention
This invention relates to a method for the purification of dioxydiphthalic anhydrides and to novel dioxydiphthalic esters useful in the purification method. Dioxydiphthalic anhydride is useful in the preparation of polyimides in a known manner by polycondensation with a suitable diamine.
2. Prior Art
Polyimides prepared from dioxydiphthalic anhydrides are useful in various industrial applications, particularly in the electronics field where purity of the polyimide is of paramount importance. In the production of dioxydiphthalic anhydride by methods known in the art (see for example, U.S. Pat. Nos. 5,089,631 and 5,003,086), a dihalophthalic anhydride of the formula ##STR1## where Hal is F, Cl, Br, or I, is reacted with an alkali metal compound such as KF or K.sub.2 CO.sub.3. The crude reaction product typically contains residual salts such as K.sub.2 CO.sub.3 and KCl (or other alkali metal halides, depending on the particular dihalophthalic anhydride reactant employed).
Dioxydiphthalic acid may be characterized by the formula: ##STR2##
The preparation of dioxydiphthalic acid and the reaction thereof with diazomethane to yield the tetra-methyl ester is reported by Govindachari et al, in the Indian Journal of Chemistry, Vol. 5, March 1967.
U.S. Pat. No. 5,166,404 to Stults et al discloses the preparation of dioxydiphthalic acid esters by reaction of dioxydiphthalic anhydride with an alkanol.
U.S. Pat. No. 3,356,691 to Gaertner discloses a process for the purification of crude benzophenone tetracarboxylic dianhydride by treating with a low boiling alcohol to dissolve impurities and filtering the mixture to separate the alcohol containing dissolved impurities. The patentee teaches that alcohols may react with dianhydrides to produce acid esters. However, the patentee, whose invention is a process for purifying benzophenone dianhydride, notes that the formation of esters should be avoided.